NATURE NOTES:
Animal Adaptations to Aridity

This episode of Nature Notes was originally broadcast on December 2, 2010, and was written by Megan Wilde of the Chihuahuan Desert Research Institute.

Your lips are chapped. Your hair unruly. Doorknobs greet you with an electric shock. The dry season has arrived in the Chihuahuan Desert region, and months of parching wind and cold separate us from summer’s rains. While the aridity at times seems a hardship for us, it poses a true survival test for wildlife. How do desert animals get enough water to live? And how do they keep from drying out?

The most obvious challenge for desert dwellers is finding water. So many animals have evolved to rarely, if ever, need a drink. Instead, some rely heavily on water produced by their own bodies. This water—called metabolic water—is generated as they digest fats, proteins, and carbohydrates in their diet.

Biologist Steven Platt teaches desert ecology at Sul Ross State University in Alpine, Texas.

Platt: Some kangaroo rats, for instance, which are kind of classic desert rodents, may never drink water in their life. They get everything they need from this diet that’s very high in carbohydrates, seeds, mostly grass seeds.

In fact, kangaroo rats can actually distinguish which seeds will produce the most metabolic water. And they deliberately store seeds in high-humidity parts of their burrows, where the contents of their pantry will stay hydrated.

Other desert animals get water more directly from their diet. Cacti, in particular, are a choice source of hydration for deer, javelina and wood rats. That’s why you’ll often see prickly pear pads and spindly cholla stems missing chunks of their succulent green flesh; they’ve been stripped by thirsty animals.

Predators can meet some or all of their water needs by eating other animals. In one experiment, roadrunners were deprived of water and fed only mice. These hardy birds survived, getting most of their water directly from water in the mice, and the rest from metabolic water as they digested them.

But, Platt explains that not all desert dwellers can get enough water from their food alone and must occasionally take a drink. Even some animals that can survive entirely off water in their diet may still drink water when it’s available.

Platt: Some animals definitely have to get water from somewhere. They’ll often travel long distances to get water. And that’s why springs and seeps are extremely important.

Typically only animals that are able to move long distances—such as birds and larger mammals—depend on such free water, and they’re well-adapted to finding it. Take coyotes, for example. When surface water isn’t available, coyotes will actually dig their own wells as deep as a meter into arroyos to try and quench their thirst.

But finding water is only one challenge. Keeping it is another in the dry desert air, which soaks up moisture like a sponge. Platt says one way animals cope is avoiding arid conditions as much as possible, by being nocturnal or living underground.

Platt: For instance, kangaroo rats spend a lot of time underground in these big burrows. Where typically compared to outside the burrow the humidity is relatively high and constant rather than fluctuating.

Kangaroo rats demonstrate another water-saving adaptation. A lot of water can be lost flushing salts and other wastes from the body as urine. So kangaroo rats excrete an extremely concentrated form of urine. Desert reptiles go a step further and excrete wastes in a dry form called uric acid.

Exhalation can be a major source of water loss too. So, Platt says, many animals have a special nasal cooling system that traps and recaptures moisture in their breath, before the dry desert air can steal this precious liquid away.

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